Volatility of Secondary Organic Aerosol from β-Caryophyllene Ozonolysis over a Wide Tropospheric Temperature Range

We investigated secondary organic aerosol (SOA) from β-caryophyllene oxidation generated over a wide tropospheric temperature range (213–313 K) from ozonolysis. Positive matrix factorization (PMF) was used to deconvolute the desorption data (thermograms) of SOA products detected by a chemical ionization mass spectrometer (FIGAERO-CIMS). A nonmonotonic dependence of particle volatility (saturation concentration at 298 K, C298K*) on formation temperature (213–313 K) was observed, primarily due to temperature-dependent formation pathways of β-caryophyllene oxidation products. The PMF analysis grouped detected ions into 11 compound groups (factors) with characteristic volatility. These compound groups act as indicators for the underlying SOA formation mechanisms. Their different temperature responses revealed that the relevant chemical pathways (e.g., autoxidation, oligomer formation, and isomer formation) had distinct optimal temperatures between 213 and 313 K, significantly beyond the effect of temperature-dependent partitioning. Furthermore, PMF-resolved volatility groups were compared with volatility basis set (VBS) distributions based on different vapor pressure estimation methods. The variation of the volatilities predicted by different methods is affected by highly oxygenated molecules, isomers, and thermal decomposition of oligomers with long carbon chains. This work distinguishes multiple isomers and identifies compound groups of varying volatilities, providing new insights into the temperature-dependent formation mechanisms of β-caryophyllene-derived SOA particles.

at 213 K, β-caryophyllene was lost to the wall too quickly to be detected and did not lead to significant particle formation after the first ozone addition.Thus, β-caryophyllene was added into the chamber a second time, now in the presence of ozone which then led to the formation of SOA.
Note that the excess of ozone was lower for the experiment at 273 K but still about 50 ppb of ozone were available after all β-caryophyllene was consumed.Therefore, we believe the chemistry in this experiment is still comparable to those done at the other temperatures.
We note that the impact of the different RH on our results cannot be completely excluded due to potential condensed phase reactions.However, due to the increasing RH with decreasing T, the potential impact of relative humidity was minimized 3,4 , except for the coldest experiment (213 K).

Section S2. Discussion of the limitation of offline FIGAERO-CIMS analysis in this work
The filter setup and all instruments were installed in the laboratory outside the chamber at room temperature (295±2 K).To minimize the effect of temperature differences between the chamber temperatures (213 K, 243 K, 273 K, 298 K, 313 K) and room temperature on the gas-phase chemical composition, the sampling line to the CIMS was partially insulated and the residence time in the Teflon sampling line was only 2 s.Hence, a significant particle evaporation can be excluded.The residence time in the stainless-steel line for filter sampling was about 1 s.The sampling time of each filter was typically 5-10 minutes plus a few minutes of handling time before the sample was stored in the freezer (-30 °C).The residence time in the stainless-steel sampling line to the AMS was 19 s, very likely allowing the sample air to reach room temperature before entering the instrument.Hence, we cannot fully exclude some particle evaporation and there could be an underestimation of the more volatile fraction of the particles especially in the AMS measurements.
However, high particle viscosity significantly hinders the evaporation of (semi-)volatile compounds from the particle phase.The viscosity of β-caryophyllene SOA particles was found to be in the range between 1.3 × 10 3 and 2.4 × 10 7 Pa•s depending on the relative humidity, suggesting β-caryophyllene particles were in a semisolid state 4 .Using a parameterization approach 6 , we estimated the particles formed at 213-273 K in this study to be also in a semisolid state.Thus, the diffusivity of β-caryophyllene SOA particles can be assumed to be too low to allow substantial evaporation of particulate compounds during the sampling process.

Section S3. Wall losses (see Gao et al., ACP 2022 5 )
Wall losses of particles and semi-volatile trace gases were calculated with the aerosol dynamic model COSIMA 1,7 and used to correct the SOA yields published already 5 .Particle losses contributed typically 6% or less to the total SOA mass.Due to the large size of the simulation chamber, and the relatively low vapor pressures of the β-caryophyllene oxidation products and the generally high molecular weights of the product spectrum, the diffusion-limited wall loss of gases is of minor importance compared to the condensation onto the organic particle phase.Thus, the losses via the gas phase contribute less than 1 to 6% to the total SOA mass with the highest value at 313 K.At low temperatures (e.g., 213 K and 243 K), besides HOMs, most products are in (or below) ELVOC ranges (see the section "Volatility determination and comparison from different methods" in the manuscript).However, other products with high molecular weights, e.g., C14H24O5-6, C15H24-26O3-5 and C30H38O5, can still be detected in both gas and particle phases.Thus, we think the effect of wall loss of HOM on the particle-phase chemical composition is limited.Therefore, while the wall losses may differ within this range between the experiments, they cannot explain the distinct changes in composition.

Section S4. Three volatility estimation methods
Formula method.In this work, we predicted the * of each molecule from its elemental composition applying a parameterization using molecular corridors 8,9 .The method is the so-called formula method, and can be expressed as 9 : where , are the number of carbon and oxygen atoms in a molecule, and , , , are the parameters.For CHO species 9 , the values are: =22.66, =0.4481, =1.656, and = -0.7790,respectively.
Tmax method.During the thermal desorption in the FIGAERO, individual molecules with varying volatilities have different thermal responses.The Tmax correlates to the effective Vp of organic species in a complex SOA mixture 10 .By measuring the correlation between the Tmax and the vapor pressure of organic species with known Vp, the * of all the detected molecules can be estimated [10][11][12][13] .Here, we calibrated this relationship using eight carboxylic acids which were dissolved in methanol and deposited on the filter with a syringe.The calibration parameters determined are applied to convert the Tmax value of each detected ion to a corresponding saturation vapor pressure.
PMF factor method.In the PMF factor method, the SOA volatility is not estimated from the individual ions, but from volatility groups (i.e., from the PMF factors).The Tmax value of each factor thermogram was converted to a * value using the same calibration parameters as for the Tmax method.Note that with the PMF factor method, the thermogram signal of a single ion can contribute to multiple factor thermograms (and thus volatility groups) if it, e.g., contains a monomer compound and a thermally decomposed dimer.In contrast, the Tmax of the thermogram of a molecule or the formula method assigns only one volatility to one detected molecule.

Section S5. Volatility calibration based on the desorption temperature maximum (Tmax) in the FIGAERO-CIMS thermograms
We calibrated the Tmax -Vp relationship of the FIGAERO-CIMS instrument using eight carboxylic acids with known saturation vapor pressures (Vp) (Table S2).We choose these compounds to partially cover the vapor pressures of the β-caryophyllene oxidation products.The eight carboxylic acids were mixed in mass mixing ratios, as shown in Table S2.We used three calibrant solutions:  * values estimated with a group contribution method 20,21 .S2).The x-axis error bars values refer to the range of Tmax of each acid in other mixtures (a) and (c).The βcaryophyllinic acid value is marked with black and was not included in the fit.

Section S6. Comparison of elemental composition and oxidation state from the measurements of FIGAERO-CIMS and HR-AMS
As shown in Figure S3, the O:C ratio and OSC from FIGAERO-CIMS measurements were slightly higher than those from HR-AMS measurements for all five formation temperatures.Both measurements showed increases in O:C ratio and OSC of particles with increasing experiment temperatures.The H:C ratios from FIGAERO-CIMS data were generally lower than values obtained from HR-AMS measurements.The discrepancy was interpreted as being linked to the higher sensitivity towards oxygenated molecules with large polarity when using iodide ionisation in CIMS, while HR-AMS detected the bulk particles including low-oxygenated species.Figure S4.The contribution of C15H24O3 in both gas and particle phases for all SOA formation temperatures.Blue diamonds refer to the gas phase, while red diamonds represent the particle phase.

Section S7. Detailed description of PMF analysis of thermograms
As showed in Figure S5, the two SOA samples in the cold cases are resolved by a similar factor pattern, indicating similar SOA formation processes at 213-243 K. Factor C1 (factor Tmax of 60 °C) and C2 (factor Tmax of 85 °C) are dominated by C15H26O4 and C15H26O5, respectively.Those two molecules are the second most abundant monomers after C15H24O3 and C15H24O4 in the cold case 5 .Factor C4 is dominated by the dimer C30H48O5.Another dimer factor for the cold case, C3 has larger contributions of dimers with higher oxidation states (signal-weighted OS of -1.0) and correspondingly has lower volatility as indicated by the higher factor Tmax (105 °C) than C4 (OS = -1.2,factor Tmax = 95 °C).The Tmax of the dimer factors is higher than the monomer factors as expected due to the longer carbon chain despite their lower mean factor OS .In the higher Tmax region, C5 (Tmax = 120 °C) shows contributions of both dimeric and apparently monomeric compounds.These apparent monomers may be either thermal decomposition products from larger oligomers, or the monomers with low volatilities.Factor C6 has the highest Tmax (145 °C) among all cold factors, and comprises of apparently monomeric, dimeric, and trimeric compounds, and is more likely a representation of the mixture of thermal decomposition compounds from oligomers.
Different from the cold case, the two SOA samples in the warm case are resolved by a totally different factor pattern with two monomer factors (W1, W2) and a dimer factor (W3).The chemical composition shows that W2 mainly consists of highly oxygenated organic molecules (HOMs), with a signal-weighted elemental composition of C15.0 H22.6 O6.8, the highest observed OS (-0.53) and O:C ratio (0.49) among all factors, and the highest monomer factor Tmax (100 °C).Thus, W2 is categorized as the least volatile monomer factor.In parallel, as the other monomer factor at the warm case, W1 has the second highest OS and O:C ratio (-0.66 and 0.45) among all factors.One molecule containing several isomers in different volatilities, e.g., C14H22O6-7, can contribute to different factors, such as W1 and W2.The PMF analysis identifies those isomers due to a large Tmax difference of 15 °C between W1 and W2.It signifies that those isomeric monomers belong to different volatility ranges.In addition, the only dimer factor in the warm case, W3, has a high Tmax of 135 °C and is actually a mix of dimers and again either extremely low-volatile monomers or thermal decomposition products from oligomers.
In the intermediate temperature case (SOA273K), there are only small contributions of the warm and cold-temperature factors while 60% of the signal is explained by the intermediate temperature a) single acids in methanol, b) mixed acids in methanol with mass ratios simulating βcaryophyllene SOA, and c) mixed acids in methanol with mass ratios simulating monoterpene SOA.The solutions were deposited on PTFE filters using a micro syringe.The reference compounds were thermally desorbed with the same desorption program that was applied to the SOA samples.The Tmax values obtained for each carboxylic acid are then related to the Vp values available in the literature (Figure S1).Error bars of Tmax indicate the range of values obtained from measurements of solutions of individual acids and other mixture solutions.The large error bars of the Tmax suggests high uncertainties among different solutions, potentially stemming from a strong matrix effect.This confirms that different ratios of acids in a solution can induce high uncertainty of Tmax, and hence bias the determination of volatility.Thus, to get a calibrant Tmax -Vp correlation that is representative of the β-caryophyllene studied in this work, we used the Tmax data of acids in the solution (b) and applied the range obtained from Tmax values from solution (a) and solution (c) as the uncertainty (i.e., as measurement errors) in the linear fitting procedure.This definition of the measurement uncertainties leads to only one-directional error bars for some data points shown in Figure S1.A linear regression was applied to obtain calibration parameters (r 2 =0.8): log10Vp=-0.48-0.04×TmaxEq.(S2) Eq. (S2) was applied to convert Tmax values of thermograms of individual ions or factor thermograms to the corresponding Vp, and Eq.(1) was used to convert these values to saturation concentrations ( * ) using the molecular weight (MW) of the individual ion or the average MW of the PMF factor.

Figure S1 .
Figure S1.Correlation between temperatures of maximum desorption (Tmax) and the known saturation vapor pressures (VP) of seven carboxylic acids.X-axis values for dots refer to the Tmax of each acid in the mixed acids solution (b) (TableS2).The x-axis error bars values refer to the range of Tmax of each acid in other mixtures (a) and (c).The βcaryophyllinic acid value is marked with black and was not included in the fit.

Figure S2 .
Figure S2.Van-Krevelen diagram for β-caryophyllene-dereived particle bulks during the SOA generation period at all temperatures from HR-AMS measurements.Symbols are coloured by temperature and sized by the time from the first ozone addition.Larger symbols indicates the more aged particles.The carbon oxidation states ( ) are shown with red dashed lines.( = 2 O:C -H:C).Grey dashed lines indicate functionalities that dominate in the SOA particles with data points all along those lines.

Figure S3 .
Figure S3.Comparison of O:C ratio, H:C ratio, and oxidation state, OSC of SOA particles formed at 213 K, 243 K, 273 K, 298 K, and 313 K between the measurements from HR-AMS (blue diamonds) and FIGAERO-CIMS (orange diamonds).
S14factors I1 and I2.These factors differ 55 °C in Tmax to each other and contain mostly monomers and a small amount of dimers in I2.

Figure
Figure S5.A 12-factor PMF solution for β-caryophyllene SOA particles at five temperatures.(a) sum thermograms (grey lines and points) and the factor thermograms for each sample; (b) the chemical composition of each factor, show as the averaged carbon oxidation state ( ) vs the carbon number of each ion (modified Kroll diagram); (c) factor mass spectra.The symbol indicates the normalized signal intensity of each ion.Colors are the same in all panels and indicate the individual factors in (a) , (b) , and (c).

Figure
Figure S6.Q/Qexp values for the varying number of factors in PMF solutions (left), and varying fpeak in a 12-factor PMF solution (right).Identification of Q, Qexp and fpeak have been detailed described previously 22 .

Table S1 .
Compilation of experimental conditions, concentration of β-caryophyllene and O3 (reprinted with permission under the terms of the Creative Commons Attribution 4.0 CC BY License 5 .Copyright 2022 L. Gao).

Table S2 .
Compounds used for the volatility calibration and saturation vapor pressures from literature.

Table S3 .
Compilation of O:C ratios, H:C ratios, , derived from FIGAERO-CIMS measurements and the Tmax of sum thermograms.